Book Description

With the increasing power and thermal limits in the computing industry, energy-efficient computing has become an urging necessity. Therefore, a surge of interest has been recently given to the concept of Near-Threshold Computing (NTC) as a potential candidate to realize energy-efficiency in computations. By operating at supply voltages near the transistor’s threshold voltage, NTC promises significant energy savings with moderate performance loss, which can be compensated for through parallelism. However, NTC faces a few challenges that hinder its wide adoption. On top of these challenges are the harsh specifications required for the power management and delivery units. Specifically, a power converter in an NTC system is required to achieve high efficiency at high current densities and low output voltages while seamlessly integrated on-chip, which are all contradicting specifications. To tackle the problem of energy-efficient computing, this research work addresses the challenges of NTC, with focus on power delivery. To do so, first, the target application of NTC is investigated to acquire the basic understanding of its challenges, opening doors for innovations and solutions for these challenges. Based on this understanding, which reveals the importance of power delivery for NTC and defines the requirements on power converters, most of the work in this thesis will focus on Switched-Capacitor (SC) power converters, which are found to be the most suitable type of converters for NTC. Therefore, a detailed study and literature review of SC converters is carried out. This study provides an in-depth understanding of SC converters operation, mechanisms, and challenges. Specifically, it is demonstrated that the most advantageous characteristic of SC converters is their compatibility with CMOS integration, while the most challenging aspect is their limited current density. Consequently, this thesis sets forth to address this challenge and proposes two solutions to boost the current density of SC converters, and thus, offering feasible power converter architectures for NTC. The first solution proposed in this thesis focuses on the control loop of SC converters. Unlike regular control loops, which often utilize frequency, capacitance, or conductance modulation, the proposed technique combines all three control knobs. The combination of these parameters allows for ripple reduction without sacrificing current density, and thus, effectively increases the converter’s density. Furthermore, this combination of parameters maintains the efficiency near its peak across a wide range of load currents, which is another relevant feature for NTC. The second solution introduces the concept of resonant gate drivers to SC converters, increasing the converter efficiency with no impact on current density. This solution is implemented in 45 nm SOI technology and fabricated for validation. The measurement results demonstrate a 70% efficiency at 1 A/mm2 current density and 0.4 V output voltage, which is a new efficiency/current-density record in the near-threshold range. In summary, as a potential solution to the problem of energy-efficiency in computations, NTC and its challenges are investigated. To address its most critical challenge of power delivery, SC converters are studied and circuit techniques are proposed to boost their current density and offer a feasible power delivery for NTC applications.